Surface sizing, as it relates to paper manufacture, is the application of a non-pigmented coating to the surface of a paper web to improve the smoothness and tensile strength of the paper for subsequent coating or printing, as well as to enhance the grease resistance of the paper.
Starch (which is produced from corn, waxy maize, tapioca, wheat, potato, and rice) is the largest volume product used commercially for surface sizing of paper. Other hydrocolloids which may be used either alone or in combination with starch include polyvinyl alcohol, carboxymethyl cellulose, wax emulsions, and alginates. It is well known that starch covers the paper surface very irregularly, and a continuous film cannot be easily applied unless a high concentration of the starch is used. Typical concentrations range from 6-12%, depending on the paper qualities desired. The starch is mixed with water, heated to swell the starch granules and solubilize amylose molecules, and the dispersion cooled to form a gel or paste. Because of the tendency for native or unmodified starch to retrograde or increase in viscosity following the normal cooking process, chemically modified or reduced-viscosity starches are generally used in paper sizes. These include oxidized, cationic, hydroxyethyl ether derivatives, and enzyme-converted starches.
It would be of advantage to have a size which had good film forming properties, such that the size could be applied in an even, non-porous coating that would permit proper sizing of the paper with the optimum quantity size and would also allow control of paper penetration by the size.
Combinations of gellan gum and starch have been disclosed in the art. For example, Baird, et al, Bio/Technology, Nov. 1983, page 781, teach that it may be desirable to use gellan gum in combination with modified starches to obtain optional product texture and stability. Kang, et al, Some Novel Bacterial Polysaccharides of Recent Development, page 240, teach that gellan gum may be used as a structuring agent to replace or partially replace the starch. Sanderson et al, Food Technology, Apr. 1983, teach at page 66, Table 4, a starch jelly formulation containing 6.56% starch and 0.2% gellan gum; at page 68, FIG. 8 amylograph for a 4.8% starch/0.2% gellan gum blend; and at page 68, the advantages of combining starch and gellan gum in pie fillings and puddings. U.S. Pat. No. 4,517,216, Table 1--1 discloses blends of 0.52% gellan gum and 0.25% corn starch.